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ESA's quartet of space-weather watchers, Cluster, has discovered vortices of ejected solar material high above the Earth. The superheated gases trapped in these structures are probably tunnelling their way into the Earth's magnetic 'bubble', the magnetosphere. This discovery possibly solves a 17-year-mystery of how the magnetosphere is constantly topped up with electrified gases when it should be acting as a barrier.

This figure shows a three-dimensional cut-away view of Earth's magnetosphere. The curly features sketched on the boundary layer are the Kelvin-Helmholtz vortices discovered by Cluster. They originate where two adjacent flows travel with different speed. In this case, one of the flows is the heated gas inside the boundary layer of the magnetosphere, the other the solar wind just outside it. The arrows show the direction of the magnetic field, in red that associated with the solar wind and in green the one inside Earth's magnetosphere. The white dashed arrow shows the trajectory followed by Cluster. Credit: H. Hasegawa (Dartmouth College)

The Earth's magnetic field is our planet's first line of defence
against the bombardment of the solar wind. The solar wind itself is
launched from the Sun and carries the Sun's magnetic field throughout
the Solar System. Sometimes this magnetic field is aligned with Earth's
and sometimes it points in the opposite direction.

When the two fields point in opposite directions, scientists understand
how 'doors' in Earth's field can open. This phenomenon, called
'magnetic reconnection', allows the solar wind to flow in and collect
in the reservoir known as the boundary layer. On the contrary, when the
fields are aligned they should present an impenetrable barrier to the
flow. However, spacecraft measurements of the boundary layer, dating
back to 1987, present a puzzle because they clearly show that the
boundary layer is fuller when the fields are aligned than when they are
not. So how is the solar wind getting in?

Thanks to the data from the four formation-flying spacecraft of ESA's
Cluster mission, scientists have made a breakthrough. On 20 November
2001, the Cluster flotilla was heading around from behind Earth and had
just arrived at the dusk side of the planet, where the solar wind
slides past Earth's magnetosphere. There it began to encounter gigantic
vortices of gas at the magnetopause, the outer 'edge' of the
magnetosphere.

"These vortices were really huge structures, about six Earth radii
across," says Dr Hiroshi Hasegawa, Dartmouth College, New Hampshire who
has been analysing the data with help from an international team of
colleagues. Their results place the size of the vortices at almost
40000 kilometres each, and this is the first time such structures have
been detected.

These vortices are known as products of Kelvin-Helmholtz instabilities
(KHI). They can occur when two adjacent flows are travelling with
different speeds, so one is slipping past the other. Good examples of
such instabilities are the waves whipped up by the wind slipping across
the surface of the ocean. Although KHI-waves had been observed before,
this is the first time that vortices are actually detected.

When a KHI-wave rolls up into a vortex, it becomes known as a 'Kelvin
Cat's eye'. The data collected by Cluster have shown density variations
of the electrified gas, right at the magnetopause, precisely like those
expected when travelling through a 'Kelvin Cat's eye'.

Scientists had postulated that, if these structures were to form at the
magnetopause, they might be able to pull large quantities of the solar
wind inside the boundary layer as they collapse. Once the solar wind
particles are carried into the inner part of the magnetosphere, they
can be excited strongly, allowing them to smash into Earth's atmosphere
and give rise to the aurorae.

Cluster's discovery strengthens this scenario but does not show the
precise mechanism by which the gas is transported into Earth's magnetic
bubble. Thus, scientists still do not know whether this is the only
process to fill up the boundary layer when the magnetic fields are
aligned. For those measurements, Hasegawa says, scientists will have to
wait for a future generation of magnetospheric satellites.

Cluster is a mission of international co-operation between ESA and
NASA. It involves four spacecraft, launched on two Russian rockets
during the summer of 2000. They are now flying in formation around
Earth, relaying the most detailed ever information about how the solar
wind affects our planet in 3D. The solar wind is the perpetual stream
of subatomic particles given out by the Sun and it can damage
communications satellites and power stations on Earth. The Cluster
mission is expected to continue until at least 2005.

The ongoing archiving of the Cluster data (or Cluster Active Archive)
is part of the International Living with a Star programme (ILWS), in
which space agencies worldwide get together to investigate how
variations in the Sun affect the environment of Earth and the other
planets. In particular, ILWS concentrate on those aspects of the
Sun-Earth system that may affect mankind and society. ILWS is a
collaborative initiative between Europe, the United States, Russia,
Japan and Canada.

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